P0505 means Idle Air Control System Malfunction. The computer cannot control idle speed as commanded. On older engines this means the idle air control (IAC) valve; on electronic throttle engines it involves the throttle body. It is a generic OBD-II code, so the definition is the same on any make, but the most likely cause varies by engine; confirm with a scan tool and the service information for your vehicle before buying parts.
What P0505 means
Most vehicles built in the last two decades use electronic throttle control instead of a throttle cable: the accelerator pedal carries two or three position sensors, and the throttle body has a motor and two position sensors of its own. The computer compares the redundant sensors constantly, and any disagreement triggers reduced-power mode as a safety measure. Older engines with a throttle cable control idle with a separate idle air control valve.
P0505 symptoms
- Idle too high, too low or hunting
- Stalling when coming to a stop
- Hard starting
Common causes of P0505
- Carbon-clogged IAC valve or throttle body
- Failed IAC valve
- Vacuum leaks
- Wiring problems
How to diagnose P0505
- Clean the throttle body and IAC passages
- Check for vacuum leaks
- Command the IAC with a scan tool and watch idle speed respond
- Inspect wiring
Can you drive with P0505?
Usually, but stalling at stops can be dangerous. Repair soon.
P0505 tips
On older engines with an idle air control valve, carbon in the valve and its passage is the classic cause; cleaning often restores idle control. On electronic throttle engines, carbon around the throttle plate does the same thing.
Will P0505 clear itself?
If the fault stops, the computer turns off the check engine light by itself after several drive cycles without seeing it again, and the code stays in memory as history for a while. Clearing P0505 with a scan tool after a repair is fine, but if the cause is still there the code returns, sometimes within one drive. Disconnecting the battery also clears codes on most vehicles, but it erases learned fuel trims and idle settings as well.
Related codes
| Code | Meaning |
|---|---|
| P0506 | Idle Air Control System RPM Lower Than Expected |
| P0507 | Idle Air Control System RPM Higher Than Expected |
| P0171 | System Too Lean (Bank 1) |
Guides that help
When it is more than a sensor
Throttle and idle codes are fixed at the throttle body, the pedal, a sensor or a leak. If those check out and the engine still runs poorly, a compression test separates a control problem from a worn engine; compare engines by code on our engines page.
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